The present invention relates to an automated sample delivery system for introducing a fluid sample into the sample receiving aperture on an analytical instrument, and, sequentially delivering aliquots of sample to the sample aperture from a single sample container.
In an analytical instrument such as an atomic absorption spectrophotometer, the sample, usually an aqueous solution, is introduced into a high temperature atomizer where it is dissociated into its component atoms. Because dissociated atoms in the ground state will absorb radiation at a characteristic wavelength, a comparison of the amount of radiation introduced into a sample at that wavelength with the percent transmission reveals the amount absorbed, from which can be calculated the concentration of the analyte of interest.
Early Atomic Absorption (AA) instruments typically used a flame to atomize the analyte elements. Detection limits for flame AA range from tens of parts per million (ppm) to subparts per billion (ppb). Electrical discharge atomizers are also known. State-of-the-art AA instruments today, however, generally use a graphite furnace for atomization. With a graphite furnace, the detection limits for many elements are lowered (improved) by 10 to 1,000 times over the prior flame atomizers. This fact makes graphite furnace AA (GFAA) one of the very few analytical techniques capable of detecting impurities in the ppb to sub-ppb range.
However, GFAA has some disadvantages. A large power supply is required to fire the furnace. Consequently, the instrument is large and expensive, both in terms of purchase price and operation cost. The graphite furnace tubes wear out and must be replaced after as few as about 70 to 200 firings, and recalibrating the instrument also contributes to shortening the lifetime of the tubes. Furthermore, analytical sensitivity decreases as the tubes wear out, so frequent recalibrations are necessary. In addition, certain acids such as H.sub.2 SO.sub.4 compromise the integrity of the pyrolytic coating on the tubes.
In the typical AA, the sample atomizer is disposed within the pathway of the optical system, which comprises a light source such as a hollow cathode lamp for emitting a narrow band width beam of light at a wavelength characteristic of the principle or other desired absorption wavelength of an analyte to be determined. Radiation from the source passes through the atomized sample, and thereafter, light exiting the atomized sample is directed through a monochromator to a detector for determining the amount of light absorbed by the sample. From the absorption characteristics, analyte concentration can be determined.
To accommodate multiple sequential analyses, a variety of devices have been developed for delivering a portion of sample from, for example, a sample vial, to the sample receiving aperture on a graphite furnace atomizer. For example, U.S. Pat. No. 4,295,854 to Huber discloses a sample delivery apparatus which comprises a rotatable turntable containing a circular array of sample vials. A servomotor is provided to swing a lever arm between two end positions such that in a first position, an electrode on the end of the lever is inserted into one of the vials on the turntable, and in the second position, the electrode is inserted into the sample receiving aperture on a graphite furnace. In this manner, the turntable can be advanced one vial at a time so that fluid from each of the vials is sequentially communicated by way of the lever arm to the graphite furnace. Each of the vials on the turntable must be manually filled with fluid to be analyzed, inserted into the turntable, and removed for disposal following use.
Notwithstanding the development of rotatable turntable delivery systems, there remains a need in certain applications for an improved sample delivery system in an analytical instrument such as an atomic absorption spectrophotometer.
For example, in the case of periodic monitoring of a sample from a single source such as a continuous sample stream, where samples may be desirably obtained from the stream at regular time intervals, rotatable sample tray delivery systems are of limited value. Such systems require repeated manual steps such as filling each of the sample cups, insertion into the tray and removal from the tray following aspiration or other sample collection by the sample arm. In addition, the rotatable sample tray delivery systems are optimally loaded with a plurality of samples prior to commencement of an analysis cycle, but in the case of periodic monitoring from a sample stream, the samples are not available except at a predetermined time interval.
Moreover, due to the high sensitivity of AA techniques, it is especially important to minimize any possibility of contamination during sample preparation and analysis. For example, with hydroscopic samples, such as solutions containing high concentrations of sulfuric acid, the problem of contamination becomes more severe. Almost anything that is absorbed or falls into the acid will dissolve and possibly introduce error into the analysis. Conventional autosampling systems have only minimal protection against contamination, i.e., a plastic cover over the open sample cups. For this reason, conventional autosampling system would not offer much protection from contamination.
Thus, there remains a need for a sample delivery system for use in an analytical instrument such as an atomic absorption spectrophotometer, which is capable of automatically delivering quantities of sample from a continuous sample stream or other source at predetermined time intervals, which minimizes the opportunity for sample contamination, and which may be automatically interspersed with appropriate aliquots of wash or reference control solutions, as desired.